In general, performance and capacity of wireless mobile communication systems are limited by air-interface propagation-channel characteristics; such as co-channel interference, path loss, multipath fading, etc. In a cellular wireless mobile communication system, a plurality of mobile stations (MSs) located in one cell perform wireless communication with a base station (BS) that manages that cell. The BS receives uplink signals from each MS.
A signal transmitted by a first MS may act as an interference component in relation to a signal transmitted by a second MS. When there are many MSs, or some MSs transmitting at high power, in a given area (i.e., sector) of a cell, the sector is considered as having high load; and the reverse link interference for that sector is generally high. As such—in a wireless mobile communication system—reverse link admission or load control should be performed in order for a BS to receive signals from MSs in a stable manner.
In a Code Division Multiple Access (CDMA)-based cellular wireless mobile communication system, reverse link interference is measured using a Rise-Over-Thermal (ROT) index. The term “ROT” as used herein refers to a ratio of a sum of total received power to thermal noise; or a difference of the sum of total received power (RSSI), in dBm, and thermal noise, also in dBm. ROT can be represented by Equation (1):ROT(dB)=RSSI(dBm)−Thermal_Noise_Floor(dBm)  (1).Load (L) is another measure for Reverse Link loading level. In principle, ROT and L should be related to each other—that relationship being expressed as:
                    ROT        =                              1                          (                              1                -                L                            )                                .                                    (        2        )            If Y is defined as intra-cell computed load, then the actual sector load is (1+f)(Y); where f is an inter-cell interference factor. Correspondingly, ROT can be obtained from Y:
                    ROT        =                              1                          1              -                                                (                                      1                    +                    f                                    )                                ⁢                                  (                  Y                  )                                                              .                                    (        3        )            There are conventional methods to control admission by setting a fixed threshold of ROT or Load [1]. This may not apply, or may not utilize the full capacity of a deployed wireless network with multimedia services because the ROT and Load are fluctuating over a large range.
Conventional methods of Reverse Link Admission Control generally control ROT or Load level in one dimension, or as a total value—without considering that different loads can not or do not account for quality of service (QoS) metrics for new or already-initiated transmissions (i.e., calls). As wireless networks and communications system evolve to become more QoS-aware, it becomes critical that Reverse Link Admission Control differentiates new or already-initiated transmissions according to their corresponding QoS levels.
Differentiation by QoS facilitates transmissions of different types of multimedia services. Most such services can be categorized into a number of types, according to delay constraints and bandwidth requirements. As a convention in 1xEV-DO system, for example, service flows are categorized as: Expedited Forwarding (EF); Assured Forwarding (AF); and Best Effort (BE). The EF flow is delay sensitive and characterized by a low data rate; the AF flow is delay sensitive and elastic; and the BE flow is delay tolerant and elastic.
As an example, IS-856 (1xEV-DO) systems have a mechanism to control reverse link load—called Closed-loop Load Control, via direct ROT measurement. With this mechanism, ROT is measured at a sector and compared with a set ROT Threshold. If the measured value is higher than the threshold, the value of a Reverse Activity Bit (RAB) is set to 1. If the measured value is lower than the threshold, the RAB value is set to 0 or −1. The RAB value is broadcast to terminals within the sector, so as to increase or decrease data rate of each terminal. A filtered RAB (FRAB) value—measured or estimated over a period of time—indicates long-term sector loading. At a BS, a ratio of time when RAB is set to 1 may be determined—which is called BusyTimeRatio and is denoted by b.
In 1xEV-DO RevA (DOrA) system that supports QoS and multi-flow packet applications, Reverse Link Load Control is further enhanced by a TraffictoPilot (T2P) allocation mechanism—where T2P is a ratio of Traffic Channel power to Pilot Channel power. T2P allocation is sector loading dependent, and is determined by flow QoS and packet transmission rate of a mobile user. Conversely, load is function of T2P values of each terminal within a sector.
Channels in DOrA system reverse link comprise: a Pilot channel; a Data Rate Control (DRC) channel; a Data Source Control (DSC) channel; an ACK channel; a Reverse Rate Indicator (RRI) channel; and a Traffic channel. The DRC, DSC, ACK and RRI are overhead channels and always on, as long as a DO connection is established between an MS and a BS.
As a result, there is a need for an admission control system that differentiates QoS requirements and, to the greatest extent possible, utilizes and cooperates with already-existing system metrics and mechanisms—such as ROT and the load estimated via T2P and b.